1. Field of the Invention
The invention relates to an apparatus for correcting a current value of a three-phase electric motor and a correcting method, to achieve smooth rotation of the three-phase electric motor.
2. Description of the Related Art
In a related-art controller of a three-phase electric motor, resistors and amplifiers of current detectors of respective phases involve individual variations. Accordingly, outputs from the current detectors contain offset errors and/or gain errors. These errors result in a discrepancy between a set current value and a current value actually supplied to the three-phase electric motor, thereby creating an imbalance in currents of the three phases. This is a cause of generation of a torque ripple. In order to solve such a problem, JP Hei. 5-91780 A discloses a method for correcting a detected current value in the following manner.
First, one of the respective phases (U, V, and W) of the three-phase electric motor is taken as a reference phase (U). At the time of correction, offset errors (Δu, Δv, and Δw) of the current detectors for the respective phases are respectively obtained in a state where current flows in none of the phases of the three-phase electric motor. Next, a predetermined current is caused to flow only in the reference phase (U) and another phase (V). A gain error (ΔGv) of the current detector for the other phase (V) is obtained from outputs produced at this time by the current detectors for U and V phases. In addition, the predetermined current is caused to flow only in the reference phase (U) and still another phase (W). A gain error (ΔGw) of the current detector for the still other single phase (W) is obtained from outputs produced at this time by the current detectors for U and W phases. During operation of the three-phase electric motor, values (Iu, Iv, and Iw) detected by the current detectors for the respective phases are corrected with using the thus-obtained offset errors (Δu, Δv, and Δw) and gain errors (ΔGv and ΔGw) of the current detectors for the respective phases, to supply current to the three-phase electric motor in accordance with the thus-corrected detection values (Icu, Icv, and Icw) of the current detectors.
FIG. 1 is a schematic circuit diagram for explaining the correction method according to the related art. Transistors Tru1, Tru2, Trv1, Trv2, Trw1, and Trw2 are provided for the respective U, V, and W phases and constitute an inverter circuit. The transistors drive a three-phase electric motor M by means of a variable-voltage/variable-duty output, which is obtained by switching an output from a direct-current power supply V+. Ru, Rv, and Rw denote resistors, which constitute the current detectors for the respective phase. FIG. 1 shows a case where a predetermined current “i” is caused to flow only in the reference phase (U) and another phase (V) to obtain outputs of the current detectors for U phase and V phase at this time.
FIG. 2 is a diagram showing another configuration for obtaining a detected current value of each phase for the purpose of correcting the current value detected by the current detector, and showing a case where a detected current value of U phase among the three phases is obtained. Transistors Tru1 and Tru2 are transistors provided for the U-phase of the inverter circuit. A predetermined current is supplied from a constant current supply source. The current is detected by the resistor Ru constituting the current detector; amplified by an amplifier Amp; and output to a microcomputer, where the current is subjected to A/D conversion. Correction is made on the basis of the thus-converted detected current value.
Specifically, in the related art, correction is made by means of generating, for instance, a specific state where a predetermined current is caused to flow only in an arbitrary phase at the time of correcting a detected current value.